There are two basic types of photoelectrochemical photovoltaic cells. The first type is the regenerative cell which converts light to electrical power leaving no net chemical change behind. Photons of energy exceeding that of the band gap generate electron-hole pairs, which are separated by the electrical field present in the space-charge layer. The negative charge carriers move through the bulk of the semiconductor to the current collector and the external circuit. The positive holes are driven to the surface where they are scavenged by the reduced form of the redox relay molecular (R), oxidizing it: h++R→O, the oxidized form. O is reduced back to R by the electrons that re-enter the cell from the external circuit. In the second type, photosynthetic cells, operate on a similar principle except that there are two redox systems: one reacting with the holes at the surface of the semiconductor electrode and the second reacting with the electrons entering the counter-electrode. In such cells water is typically oxidized to oxygen at the semiconductor photoanode and reduced to hydrogen at the cathode. Titanium dioxide has been the favoured semiconductor for these studies.
Mesoscopic or nano-porous semiconductor materials, minutely structured materials with an enormous internal surface area, have been developed for the first type of cell to improve the light capturing efficiency by increasing the area upon which the spectrally sensitizing species could adsorb. Arrays of nano-crystals of oxides such as TiO2, ZnO, SnO2 and Nb2O5 or chalcogenides such as CdSe are the preferred semiconductor materials and are interconnected to allow electrical conduction to take place. A wet type solar cell having a porous film of dye-sensitized titanium dioxide semiconductor particles as a work electrode was expected to surpass an amorphous silicon solar cell in conversion efficiency and cost. These fundamental techniques were disclosed in 1991 by Graetzel et al. in Nature, volume 353, pages 737-740 and in U.S. Pat. Nos. 4,927,721, 5,350,644 and JP-A 05-504023. Graetzel et al reported solid-state dye-sensitized mesoporous TiO2 solar cells with up to 33% photon to electron conversion efficiences.
In 1995 Tennakone et al. in Semiconductor Sci. Technol., volume 10, page 1689 and O'Regan et al. in Chem. Mater., volume 7, page 1349 reported an all-solid-state solar cell consisting of a highly structured hetero-junction between a p- and n-type semiconductor with a absorber in between in which the p-semiconductor is CuSCN or CuI, the n-semiconductor is nano-porous titanium dioxide and the absorber is an organic dye.
EP-A 1 176 646 discloses a solid state p-n heterojunction comprising an electron conductor and a hole conductor, characterized in that if further comprises a sensitizing semiconductor, said sensitizing being located at an interface between said electron conductor and said hole conductor; and its application in a solid state sensitized photovolaic cell.
A drawback in the manufacture of nano-porous metal oxide semiconductor layers for Graetzel photovoltaic cells is the high temperature needed for making the nano-porous metal oxide semiconductor layer. This is apparently needed to obtain sufficient contact between the nano-porous metal oxide particles to create a conductive pathway for the photogenerated charges (electrons). Although the term sintering is not the appropriate one in this context, this term is often used to describe this heating process. Usually temperatures between 300 and 550° C. are applied for 15 to 90 minutes. Such high temperatures are prohibitive for making photovoltaic cells on plastic and flexible substrates. Such cells would offer a myriad of advantages for this type of photovoltaic cell.
In 1996 C. J. Barbé et al. reported in the Materials Research Symposium Proceedings, volume 431, pages 129-134, the development of a new type of solar cell based on a photo-electrochemical process with which a respectable photovoltaic efficiency of 10% could be obtained by the use of mesoproous, nanostructured films of anatase particles. They also reported on how processing parameters such as hydrothermal growth temperature during autoclaving, binder addition and sintering conditions influence the film porosity and pore size distribution of colloidal TiO2 nanoparticles and consequently affect the solar cell efficiency. Autoclaving temperatures between 200 and 250° C. were used with the average aggregate size and the average pore size increasing with increasing autoclave temperature. Films were fires at 400, 450, 500 and 550° C. at 5°/min in air to study the influence of the heat treatment temperature on the final film morphology.
WO 00/72373 discloses a method for manufacturing a nanostructured porous film electrode, the method characterized by the steps of: preparing a binder-free suspension (21) of electrode material particles (11) in a volatile suspending agent (13), said particles substantially having a size within the nanometer scale, depositing the binder-free particle suspension (21) on a substrate (22) covered with a conducting film, removing the suspending agent (31) by evaporation, and compressing the particles to form an electrically conducting and mechanically stable nanostructured porous film. The process of WO 00/72373 enables the realization of the same solar cell performance with high pressure sintered nano-porous titanium dioxide layers as with conventional high temperature sintering. This finding was confirmed in 2000 by Pichot et al. in Langmuir, volume 16, pages 5625 to 5630, and in 2001 by Lindstrom et al. in Nano Letters, volume 1, pages 97 to 100. However, although this high pressure sintering process appears to work quite well with Degussa P25, a nano-sized titanium dioxide with a mean particle size of 30 nm and a specific surface of 50 m2/g from DEGUSSA, on a glass substrate or a plastic substrate, it has been found not to work with titanium dioxide particles made by a wet precipitation process.
There is a therefore a need for a low temperature process for preparing nano-porous metal oxide semiconductor layers with nano-particles prepared by wet precipitation processes on supports.